Owing to characteristics such as small size, lightness, and low price, switching power supply devices are used as power supplies in a large number of electronic instruments. Among these, resonant switching power supply devices are widely used as power supplies for liquid crystal display devices, flat panel displays (flat screen televisions) such as plasma display panels, and personal computers, as the resonant switching power supply devices can realize low noise and high conversion efficiency.
FIG. 8 is a circuit diagram of a general resonant switching power supply device, FIG. 9 is a diagram showing the input voltage conversion ratio of voltage generated in a transformer winding in response to a change in control frequency, and FIGS. 10A and 10B are diagrams showing changes in resonant current when controlling, wherein FIG. 10A shows an operation in a correct control condition, while FIG. 10B shows an operation when the condition is such that the resonance deviates. In FIGS. 10A and 10B, the broken lines show the zero level of each signal.
A general resonant switching power supply device includes as a main circuit two switches Q1 and Q2 connected in series to either end of a direct current power supply Ed. A series circuit of a resonant capacitor Cr, a resonant inductor Lr, and a winding P on the primary side of a transformer T is connected to both ends of the high side switch Q2. Although not shown, the winding P is formed of a leakage inductor of the transformer T and an exciting inductor. The leakage inductor may be used as the resonant inductor Lr, without providing a dedicated inductor separate from the transformer T. A rectifying and smoothing circuit having diodes D1 and D2 and a smoothing capacitor Co is connected to windings S1 and S2 on the secondary side of the transformer T. An output voltage monitor circuit 10, which detects output voltage, is connected to an output of the rectifying and smoothing circuit, and the output voltage monitor circuit 10 is connected to a control and drive circuit 14 via a photocoupler 12.
The control and drive circuit 14 controls the two switches Q1 and Q2 to be turned on and off alternately, controlling the on-state time of the two switches Q1 and Q2, or the frequency, so that the output voltage detected by the output voltage monitor circuit 10 is constant, thereby stabilizing the output voltage.
As can be seen from FIG. 9, which shows a voltage conversion ratio M of voltage generated in the winding of the transformer T in response to a change in a control frequency fsw that controls the switches Q1 and Q2 to be turned on and off, the voltage conversion ratio M depends on the control frequency fsw, because of which the resonant switching power supply device controls the voltage conversion ratio M by changing the control frequency fsw. That is, the resonant switching power supply device controls energy transmitted to the secondary side of the transformer T by changing the control frequency fsw. In the drawing, f0 is a first resonant frequency, which is the resonant frequency of the series resonant circuit of the resonant inductor (or transformer primary side leakage inductor), exciting inductor, and resonant capacitor Cr, and f1 is the resonant frequency of a series resonant circuit formed of the resonant inductor Lr (or transformer primary side leakage inductor), a synthetic inductor formed by the secondary side (load side) leakage inductor and transformer exciting inductor connected in parallel, and the resonant capacitor Cr. Frequency control is generally carried out in a frequency range higher than the first resonant frequency f0. That is, the control frequency fsw is raised when there is a light load, while the control frequency fsw is lowered when there is a heavy load, thereby controlling the energy transmitted to the secondary side. In a control region wherein the voltage conversion ratio M decreases in response to a rise in the frequency, the phase of the current flowing through the winding P on the primary side of the transformer T (that is, the resonant current) is delayed with respect to the voltage in the winding P.
In the heretofore described example of the resonant switching power supply device, a minimum operation frequency is set so that the inclination of a change in the voltage conversion ratio M in response to a change in the control frequency fsw is not reversed. However, a control frequency fp at which the voltage conversion ratio M reaches a peak comes nearer to the resonant frequency fl as the load becomes heavier (the peak at the frequency f0, shown by the heavy lines in FIG. 9, corresponds to a case in which the load is zero). In the event that the minimum frequency setting is set in the vicinity of the first resonant frequency f0, the phase of the current advances with respect to the voltage of the winding P on the primary side of the transformer T when the control frequency fsw falls below the control frequency fp due to an abrupt change in the load or input voltage, or the like.
At this time, as shown in FIGS. 10A and 10B, it may happen that the resonant current is inverted during the on-state period of each of the switches Q1 and Q2. That is, in a correct control condition, the switch Q1 is turned off before the resonant current (Cr current) is inverted, as shown in FIG. 10A. However, it may happen that the resonant current is inverted before the switch Q1 is turned off, as shown in FIG. 10B. When the switch Q1 is turned off in this condition, the current that has been flowing through the switch Q1 flows into the diode connected in parallel to the switch Q1. When the switch Q2 is turned on in this condition, a reverse voltage is applied to the diode connected in parallel to the switch Q1, and a recovery current flows into that diode. As the recovery current has an extremely high temporal change rate, that is, di/dt, excessive stress is placed on the switches Q1 and Q2, leading in the worst case to element destruction. This phenomenon is called resonant deviation, and it is important to prevent this phenomenon in order to realize high power supply reliability.
When adopting a minimum frequency setting in the vicinity of f1 in an attempt to avoid an inversion of the resonant current, it is no longer possible to obtain a voltage conversion ratio M of one or more. That is, it is not possible to secure the necessary output voltage when the input voltage is low, and as the possible control range is reduced because of this, it is not desirable to adopt a minimum frequency setting in the vicinity of f1.
Technology whereby the resonant current (or switching current) is detected, and it is detected whether or not the control frequency for turning the switches on and off is beyond the lower limit of the control range from whether or not the fall (trailing edge) of a switch gate drive signal is in the vicinity of zero resonant current, is known as technology that prevents an inversion of resonant current (refer to PTL 1). When the control frequency is outside the control range, the control frequency is returned to the normal control range by changing the timing of a change or oscillation of the control frequency.
Also, a method whereby, in order to avoid an inversion of the resonant current, the detected value of the resonant current is simply compared with a threshold value voltage, and the switch is forcibly turned off when the absolute value of the resonant current becomes lower than the absolute value of the threshold value voltage, is also conceivable.
Also, there is known technology whereby the resonant current is detected, and a switch is turned off when the absolute value of the resonant current, after becoming higher than the absolute value of a first threshold value, becomes lower than the absolute value of a second threshold value whose absolute value is lower than that of the first threshold value (refer to PTL 2).